DESCRIPTION: (Adapted from the application) The goal of this project is to test and further extend the hypothesis that the hypothalamic arcuate nucleus is a novel site of cardiovascular regulation, where stimulation of alpha-2 adrenergic receptors lowers blood pressure and heart rate by the release of the proopiomelanocortin (PMOC)-derived peptides beta-endorphin and alpha-MSH which, in turn, activate distinct opiate and melanocortin receptors located in the nucleus tractus solitarii (NTS). The proposed experiments will test several aspects of this hypothesis. Using genetically modified mice lacking the various alpha-2 AR sub types, as well as alpha-2 AR sub-type-specific antagonists in rats, we will define the alpha-2 AR sub-types in the arcuate nucleus which mediate hypotension and bradycardia. Inhibition of these effects in the NTS by anti-sera against the beta-endorphin and alpha-MSH or by antagonists of their respective receptors will be taken as evidence for the intermediary role of the two peptides. We will also test whether stimulation of arcuate alpha-2 AR can facilitate the baroreceptor reflex, and thus produce depressor effects indirectly. The relative role of such an indirect effect versus direct activation of the efferent baroreflex arc will be determined by comparing the hypotensive/ bradycardic response to intra-NTS injections of beta-endorphin or alpha-MSH in control and barodenervated animals. The link between hypothalamic alpha-2 AR and beta-endorphin/alpha-MSH will be further tested by measuring be biosynthetic rate of the two peptides in isolated hypothalami from rats chronically pre treated with vehicle or with alpha-methyldopa. These latter experiments will test whether the previously reported increase in hypothalamic POMC mRNA is reflected in increased peptide synthesis following alpha-methyldopa, and whether alpha-2 AR activation can differentially affect the processing of the two peptides. The results will help our understanding of anti hypertensive drug action, and will clarify the mechanism of cardiovascular regulation by POMC peptides.